As shown in FIG. 8, illustrating the prior art, a capacity control device which provides capacity control by shifting a slide valve(V) under the pressure difference between the high pressure side and low pressure side in the compressor is previously well known as described in Unexamined Japanese Patent Application No. Sho 57-137637. As shown in FIG. 8, said capacity control device comprises said slide valve (V) which is freely slidably mounted on the compressor casing(A), and a cylinder(C) housing a piston (P) which cylinder is provided on the outside of said casing(A), said slide valve(V) being connected with the rod(R) of said piston (P), the rod end chamber (C.sub.1) and head end chamber(C.sub.2) of said cylinder (C) being communicated with the high pressure side (HP) in the compressor through communcation holes (B.sub.1, B.sub.2) respectively, a plurality of escape holes (H.sub.1,H.sub.2) being provided on said cylinder(C), low pressure side connection pipings (D.sub.1,D.sub.2) each having solenoid valves(SV.sub.1, SV.sub.2) being connected said escape holes (H.sub.1, H.sub.2). By releasing the pressure of said rod end chamber (C1) by opening said solenoid valves (SV1, SV2), said slide valve (V) shifts via said piston(P), thus providing capacity control.
Further in this construction, in order to avoid the liquid compression at the start-up of operation and relieve the starting torque, a spring(s) is provided to urge said piston(P) in the right-hand direction in FIG. 8 and position said slide valve (V) in the right-hand direction in FIG. 8 for fully opening of capacity control passage(E). Therefore, when the high pressure side (HP) and low pressure side (LP) are balanced, said slide valve(V) is located in the right-hand direction by dint of said spring(s) and said capacity control passage(E) is fully opened.
Furthermore, the right end surface of said slide valve(V) in FIG. 8 is exposed to the discharge operation side to thereby be subjected to high pressure and the left end surface is exposed to said capacity control passage(E) and subject to the low pressure.
In FIG. 8, the righthand side of casing (A), i.e., the outside part of casing (A) where cylinder (C) is arranged, is a continution of the discharge chamber and is subject to discharge pressure.
When said solenoid valves(SV1, SV2) are both closed, the rod end chamber (C1) and head end chamber(C2) are charged to the high side pressure and said slide valve(V) shifts in the left direction under the difference in pressure acting on both pressure bearing surfaces thereof and completely close said capacity control passage(E), thereby enabling 100% loading operation. Further, by successively opening said solenoid valves (SV1),(SV2), the pressure in said rod end chamber(C1) is lowered and said piston(P) shifts in the right direction overcoming a force of the difference in pressure acting on both pressure bearing surfaces of said slide valve(V) and stops at the location closing said escape holes(H1), (H2). Therefore, said slide valve (V) moves along with the movement of said piston(P) to thereby stepwise open the capacity control passage(E), thereby enabling 66%, 33% loading operation.
With the conventional capacity control mechanism shown in FIG. 8, since said valve(V) is fully opened, at the start-up, by the action of said spring(s), pressure difference between the high pressure and low pressure side sufficient to overcome the force of said spring(s) is necessary to transit from no loading operation (10% or 15% capacity) at the starting to a minimum loading operation for example 25% or 30% capacity. However, such pressure difference is not rapidly available. Therefore, the conventional mechanism has a problem of slow start-up for loading operation.
For example, in case of heat-pump type refrigeration system using a 4 way change-over valve actuated driven by the high side and low side pressure difference as an acting force, this problem may result in the failure or malfunction of said 4 way change-over valve's operation. For this reason, it is necessary, upon employment of the 4 way change-over valve, to provide additional means to ensure said valve operation under a low pressure difference condition. In order to speed up the rise in differential pressure at the start-up, the use of an oil hydraulic pump is conceivable to generate pressure higher than the discharge pressure at the start-up for applying this higher pressure on said piston(P) and thereby forcibly moving said piston(P) to thereby raise up the load. However, an oil hydraulic pump required separately is not desirable especially with a screw compressor where compactness is an important requirement, and constitutes disadvantages in respect of cost and reliability. Therefore, this idea does not provide a complete solution of said problem.
In studying the pressure distribution within the screw compressor under no loading condition (low differential pressure condition) at start-up as shown in FIG. 7, the following fact has been found. Since the discharge side and capacity control passage (E) communicate with each other via the screw rotor, the pressure should be the same all over the screw rotor. This is true statically. However, dynamically because of the refrigerant gas flow in the screw rotor, the in-process pressure (PM) in the compression process is higher than discharge pressure(PD). For instance, when measuring the groove pressure in the screw rotor at the casing, in case of suction pressure(PS) 10 kg/cm.sup.2, an in-process pressure (PM) of 11.5 kg/cm.sup.2 was obtained, which was 1.5 kg/cm.sup.2 higher than discharge pressure (PD) of 10 kg/cm.sup.2 (same as PS).
Though this value is slightly affected by the low pressure side condition and location of stoppage of slide valve, the following approximate formula applies between in-process pressure (PM) and suction pressure (PS) EQU PM.apprxeq.C.times.PS
Based upon said measured values and conversion factor 1.03, said constant (C) becomes 1.14.